This invention relates to a wireless communication system, and is particularly preferable for a cellular wireless communication system including small-scale base stations (such as femto cell base stations).
Conventionally, a cellular wireless communication system such as a system for cellular phones has employed macro cells in which one base station covers a wide area. The macro cell system has such characteristics that it reduces the number of handoffs and thus is suitable for a high-speed travel, and that the area can be extended by a small number of base stations. The macro cell system has a defect that it is difficult to overcome locally-generated blind spots such as those generated indoors.
In order to address this problem, an approach that, at a location which it is hard for radio waves from a macro cell base station to reach, a base station for providing a cell smaller than the macro cell is provided to eliminate a blind spot, has been proposed.
In particular, recently, a femto cell system in which a base station as small as to cover only a single house is provided in each household has been gaining attention, and there has been a trend of employing the femto cell system in the WiMAX and the evolution data optimized (EV-DO).
In the femto cell system, a small base station (femto cell base station) having a wireless coverage as wide as a wireless LAN is provided in each household, and the femto cell base station is connected to a cellular communication network via the Internet. Since the femto cell base station is intended for eliminating blind spots, the femto cell base stations are operated in a manner that the femto cells locally overlap the macro cells. Moreover, a frequency band used by the femto cell base station is the same as a frequency band used by the macro cell base station.
At a location where cells provided by two base stations using the same frequency overlap each other, for a mobile station connected to one of the base stations, a downlink signal from the other base station, to which the mobile station is not connected, appears as interference to a downlink signal from the base station, which is the destination of the connection. When this interference occurs in a burst manner, the reception quality of the downlink signal to the mobile station degrades. This problem is pointed out in JP 2007-129405 A.